Absorbent articles for receiving and retaining bodily discharges (e.g. urine or feces) such as disposable diapers, training pants, adult incontinence articles are well known in the art, and significant effort has been spent against improving their performance. Such improvements generally aim at addressing the primary function of such articles, namely retaining body fluids, but also at minimizing the negatives associated with wearing such articles by increasing the comfort of the wearer.
Many improvements are related to the “absorbent core” of the absorbent article. In the absorbent core the waste material is acquired by the article (picked up), then conducted away from the location of acquisition (distributed), and then stored (retained).
It is well established that reducing the thickness of the article by reducing the thickness of the absorbent core, helps to improve comfort. This, however, was always a question of balance between liquid handling performance and thickness. Also a substantial amount of cushioning was considered necessary for comfortable diapers. Finally the skilled person considered it impossible to reduce or even remove the fibrous material commonly applied in absorbent cores to a point where the modern particulate super absorbent materials would take over the biggest part or even all of the liquid acquisition and distribution functionalities previously provided by fibrous matrixes. Even if there were structures which could possibly provide all such beneficial aspects when dry, it was completely in-conceivable that this could be built into an absorbent core such that the liquid handling and comfort performance would be maintained even after the first gushes of liquid had been absorbed.
The development of absorbent cores of particular thinness has beneficial aspects, which make such a development the subject of substantial commercial interest. For example, thinner diapers are not just less bulky to wear, conform better to the body and fit better under clothing, they are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
As indicated, the ability to provide thinner absorbent articles such as diapers has been dependent on the ability to develop relatively thin absorbent cores that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of absorbent polymers often referred to as “hydrogels,” “super absorbents” or “hydrocolloid” material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 and U.S. Pat. No. 3,770,731 that disclose the use of such absorbent polymers (hereafter referred to as any of the following: hydrogel forming absorbent polymers, super absorbents, super absorbent polymers or SAPs, absorbent gel materials or AGMs). Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these SAPs to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 and U.S. Pat. No. 4,935,022 that disclose dual-layer core structures comprising a fibrous matrix and SAPs useful in fashioning thin, compact, non-bulky diapers.
SAPs are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly and homogeneously cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N,N′-methylene-bisacryl-amide, trimethylol-propane-triacrylate or triallyl-amine. These slightly cross-linked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxyl groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
It is often desirable to increase the stiffness of SAP particles. Typically, this is done by surface cross-linking, wherein the carboxyl-groups exposed on the surface of the SAP particles are additionally cross-linked to each other. Other methods to increase the stiffness comprise coating the SAP particles. Such coatings are known for example from WO 97/12575, which discloses absorbent materials comprising absorbent gelling particles and a polycationic polymer covalently bonded to the absorbent gelling particles. European patent application EP 493 011 A2 refers to an absorbent matter comprising a water-absorbent resin particle having an acidic group on the surface, a cellulose fiber and a cationic high-molecular compound having a weight-average molecular weight of 2,000 or more. Further, WO 03/043670 discloses superabsorbent particles with a shell comprising cationic polymer cross-linked by the addition of cross-linker and adhered to the hydrogel-forming polymer by applying a coating solution containing both a cationic polymer and cross-linker.
However, the development and improvement of SAPs has so far mainly focused on use of the SAPs for final storage of liquid in the absorbent core. Consequently, it would be desirable to have SAPs, which are especially suitable for fluid acquisition and distribution in the fluid acquisition zone of the absorbent core. By replacing the fibers used in the fluid acquisition zone of prior art absorbent cores with SAPs, it would be possible to further reduce the bulk and thickness of the absorbent core.
A problem in developing SAPs for use in the fluid acquisition zone is, that the demands on the physical and chemical properties of these SAPs differ considerably from the requirements for use in the fluid storage zone. For example, it is desirable that SAPs in the fluid acquisition zone are able to quickly acquire via capillary pressure and temporarily hold fluids in voids between the SAP particles, especially in “gush” situations. The focus is not primarily on SAPs with high capacity, but it is important that a hydrogel-bed formed from SAPs has high porosity and permeability in order to be able to provide enough interstices between the swollen SAPs.
It would also be desirable to have SAP particles for use in absorbent cores, especially in the fluid acquisition zones, which have a high Free Swelling Rate (FSR). The FSR is related to the surface area of the SAP particles, and thus to their particle size, particle shape and morphology of the particles (e.g. a porous SAP particle versus a non-porous SAP particle). SAP particles having a high surface area typically also have a high FSR, which means they are able to quickly absorb liquids. The ability to quickly absorb liquids is especially important in the fluid acquisition zone of absorbent cores to create sufficient void volume. It is especially desirable to have SAP particles for use in the fluid acquisition zone, which are highly saturated already after the fist gush of liquid.
Moreover, it would be desirable to be able to provide an absorbent core comprising an acquisition zone having a relatively high concentration of SAP particles with relatively high porosities, and high permeability, and in a matrix that provides sufficient wet integrity such that its capability for acquiring and transporting fluids is not substantially reduced or minimized, even when subjected to normal use forces.
It would also be highly desirable to be able to use SAPs in these absorbent cores, especially in the fluid acquisition zones, that, when swollen by body fluids, continue to have a good wet integrity and high porosity such that the void volume per unit weight of absorbent polymer relatively high and preferably is close to that of an air-laid fibrous web, such as have been used in fluid acquisition zones of prior art absorbent articles.
Hence it is an object of the present invention to provide absorbent articles having an improved fit especially by reducing their thickness but also when being loaded, together with good fluid handling performance, especially by using materials having particulary suitable fluid distribution properties when dry and during progressive saturation with liquids.
It is a further object of the invention to achieve this by using super absorbent polymers particles.